1. Field of the Invention
The present invention relates to a semiconductor device and a method for making the same. More specifically, the present invention relates to a semiconductor device formed into a small package having a lower surface formed with a plurality of external terminal portions arranged in a matrix pattern, and a method of making such a semiconductor device.
2. Background Art
A semiconductor device commonly called BGA (Ball Grid Array) package type or area array package type comprises an insulated substrate having a lower surface formed with a plurality of external terminal portions arranged in a matrix pattern. On an upper surface of this substrate, a semiconductor chip is mounted. Conventionally, the insulated substrate is a rigid substrate typically made of a glass epoxy resin. This rigid substrate has a lower surface formed with a plurality of external terminal portions typically made of solder arranged in a matrix pattern. On the other hand, an upper surface of the substrate is formed with wiring patterns for establishing electrical connection respectively with the plurality of external terminal portions. The external terminal portions are formed by first making the plurality of holes arranged in the matrix pattern on the insulated substrate, then forming the wiring pattern so that each of the holes is reached by a portion of the wiring pattern, forming the plurality of solder balls correspondingly at respective holes, and then heating the solder balls so that part of each solder ball melts to connect with the corresponding portion of the wiring pattern. Through such operations, there is formed the matrix of bump external terminal portions projecting out of the lower surface of the insulated substrate by a predetermined amount and electrically connected with the wiring patterns formed on the upper surface of the insulated substrate. With such an arrangement, the plurality of external terminal portions can be compactly disposed in the matrix pattern on the lower surface of the package. Thus, the package can be compact, and therefore the semiconductor device of this type is suitable to a high-density mounting on a mother substrate.
However, the conventional semiconductor device described above has following problems since the semiconductor chip is mounted to the insulated substrate which is rigid and is made of glass epoxy resin.
First, the thickness of the rigid substrate takes up a considerable part of the total thickness of the semiconductor device. This limits further miniaturization of the semiconductor device in terms of the thickness as well as limiting further reduction in weight.
Second, the rigid substrate made of glass epoxy resin has a coefficient of thermal expansion which is very different from that of the semiconductor chip. Thus, when the semiconductor device is being mounted to the mother substrate, heat applied during the operation may cause excessively large differences in expansion, destroying mechanical or electrical connection between the semiconductor chip and the insulated substrate.
Third, since the rigid substrate has a considerable thickness, it is difficult to further increase disposition density of the external terminal portions formed in the lower surface of the rigid substrate. Specifically, each of the external terminal portions is formed as described earlier by making a hole in the insulated substrate, filling the hole with a ball of solder, and then melting the solder to bond with the wiring pattern formed in the upper surface of the substrate. Hence, in order to make the external terminal portion projecting out of the lower surface of the insulated substrate by a desired amount, the hole in the insulated substrate must be made large enough, and the solder ball to fill the hole must be accordingly large. This limits the disposition density of the external terminal portions, limiting the number of usable terminals in the semiconductor chip for making the semiconductor device, making it impossible to appropriately meet the demand for high-density mounting, as well as making manufacture more costly due to a large consumption of solder.